It's the little things that count. You can "rev" the engine on the Merc without it being in gear. I've never needed to but it's easily done.

I don't know for sure as I don't own a Merc, I was going by what the engine mechanic and salesman at Maritime Solutions/Inflatable Experts in Annapolis told me, perhaps I misunderstood him or he was incorrect?

In the first link/video, at about 2:30 he shifts the engine into forward and reverse, it appears to me you cannot rev the engine without it being in gear, except using the choke? 2011 Mercury 9.9 4-stroke (Review) - YouTube